This invention relates to a method and system for calibrating a wireless location system (WLS) to enable the system to make highly accurate differential measurements such as time difference of arrival (TDOA) and frequency difference of arrival (FDOA).
Wireless location systems are becoming increasingly important. An example is disclosed in commonly assigned U.S. Pat. No. 5,719,584 to Otto, the disclosure which is incorporated by reference in its entirety. Many wireless location systems use time difference of arrival (TDOA) calculations to determine a set of possible locations of a transmitter of a signal. The location is mathematically determined, as a hyperbola in two dimensions and a hyperboloid in three dimensions, from the known locations of two receivers and the difference in the measured time of arrival (TOA) of the signal at those two receivers. These systems use a variety of methods to measure the TOA of a signal at a receiver. All share, however, the common requirement that the clocks must either be synchronized or the offset between the clocks measured and a correction applied. The correction may be applied either directly to the clocks or mathematically to the calculations of time differences.
In many wireless location systems an attempt is made to synchronize the clocks at the receivers. A popular method is to use a clock source at each of the receivers that is synchronized to the global positioning system (GPS) transmissions. These systems often use an oscillator with good short-term stability to drive the clock and apply a correction based on the filtered difference between a received GPS timing signal, which has good long term stability, and the clock. These systems require a GPS receiver and GPS antenna with a clear view of multiple GPS satellites.
Despite significant recent advances in these systems, the root-mean-square (RMS) difference in time between two such clocks may be as high as many tens to hundreds of nanoseconds resulting in significant errors in location estimates, particularly when geometric dilution of precision (GDOP) is significant. Although synchronization of the clocks in this manner may reduce TOA measurement errors due to clock offsets, the system must also be carefully designed and calibrated to ensure that delays in the receiver processing (both the signal processing chain and the timing distribution chain) are fixed and properly taken into account.
Some prior art systems use external calibration techniques to correct clock offsets and to correct for other variations in the receivers that may introduce TOA (and, therefore TDOA) measurement errors or errors in other measured parameters such as frequency of arrival (FOA). In these systems, receivers at known locations measure certain parameters of a signal transmitted by a stationary reference transmitter at a known location. The measured parameters are then communicated to a common point where a processor calculates offsets or adjustments that are either used to adjust one or both of the receivers or are applied to the time difference of arrival (TDOA) and/or frequency difference of arrival (FDOA) calculations.
One such prior art wireless location system as disclosed by U.S. Pat. No. 6,184,829 to Stilp, reduces instrumentation error by a calibration process where by multiple wireless transmitters, such as cellular telephones, are placed at known locations throughout the coverage territory of the wireless location system. These phones make transmissions, such as periodic registrations or page responses, in a manner similar to any other phone. Because the location and the theoretical TDOA values for any pair of receivers are known a priori, the system can determine the error in the TDOA measurements made in connection with a particular pair of receivers.
In addition, because the phones are in fixed locations and there is no Doppler shift, the theoretical FDOA value is zero. Any measured error will be caused by drifts in the oscillators at each of the receivers, changes in the characteristics of analog components (e.g., the antennas, cabling, and filters), and environmental factors. A correction is applied to the computed TDOA and FDOA values in the digital signal processing stages of the system.
These prior art external calibration systems have several limitations. Periodic transmissions must be made by reference transmitters at known locations and use system capacity that might otherwise carry normal user traffic. If a long period elapses between calibration and a subsequent TDOA and/or FDOA measurement, then the calibration may be degraded by oscillator offsets or changes in the receivers because to such things as component parameter shifts are caused by temperature. Although reducing the interval between reference transmissions improves calibration, it is at the further expense of additional system capacity. Another drawback is that multipath induced errors in the time of arrival measurements made by the receivers during calibration, i.e., when measuring the TOA/TDOA of the reference transmissions, corrupt the TDOA calibration since straight-line propagation from the reference transmitter to each receiver is presumed.
It is therefore an object of the present invention to provide a method for external calibration of wireless location systems that reduces or removes TDOA, FDOA and/or other differential measurement errors arising from many sources within the system.
It is another object of the present invention to provide a system and method for calibrating wireless location systems that is operable with reference transmitters of unknown location that may be stationary or mobile with unknown vector velocity.
It is yet another object of the present invention to provide a system and method for calibrating wireless location systems that may use normal user traffic transmissions from unknown locations by stationary or mobile transmitters of unknown vector velocity as reference transmitters thereby minimizing the system capacity used for calibration and reducing or eliminating degradation in calibration due to system drift between the instant of calibration and the instant of a measurement.
It is yet another object of the present invention to provide a system and method for calibrating wireless location systems that does not require a straight-line propagation path from reference transmitters to receivers in order to accurately calibrate TDOA or other difference measurements.
It is still another object of the present invention to provide a system and method for calibrating wireless location systems that is useful in systems using fixed, mobile or both fixed and mobile receivers.
In accordance with the present invention, a system and method determines the offsets of pairs of receivers used in making TDOA, FDOA and/or other differential measurements of signals. A transmitter at an unknown location can be is either stationary or mobile with unknown vector velocity. A plurality of fixed or mobile receivers of substantially known or determinable location (and, in the case of moving receivers making FOA measurements, of known vector velocity) receive the signal from the transmitter via multiple paths due to reflection and refraction of the signal by natural or manmade objects in the vicinity of the transmitter and/or receivers. The signal arriving at a receiver may or may not include a straight-line path signal from the transmitter to that receiver.
Each receiver measures the TOA and/or FOA or other parameter of at least one, and in some embodiments several or all, of the path signals believed not to be a straight-line path. Although not necessary for purposes of calibration, it is preferable that the receiver also measure the TOA and/or FOA of the straight-line path signal, if present, for use in the course of performing transmitter location and velocity determination which may occur coincident with calibration.
A processor is operatively connected to the plural receivers and selects a stationary natural or manmade object that is believed to have reflected or refracted the signal to each of the plural receivers and designates that object as a proxy reference transmitter, also referred to in some instances as proxy receiver because it xe2x80x9creceivesxe2x80x9d a signal and reflects or refracts the xe2x80x9creceivedxe2x80x9d signal end thus acts as a xe2x80x9cproxy reference transmitterxe2x80x9d of the signal. Hereafter, the signal reflected or refracted by that object to the plural receivers may be referred to as a proxy reference transmission.
The location of the proxy reference transmitter is either stored in a database operative with the processor or determinable from information stored in the database such as, but not limited to, aerial photographic imagery. The processor then determines the differential measurement, for each combination of receiver pairs receiving the proxy reference transmission. In the case of FDOA, because the proxy reference transmitter is a stationary object and the Doppler shift imparted by any motion of the transmitter relative to the proxy reference transmitter is common to the reflected or refracted signal at both receivers, the theoretical FDOA value is zero. Any measured error will be due to drifts in the oscillators at each of the receivers, changes in the characteristics of analog components (e.g., the antennas, cabling, and filters), and environmental factors.
The processor may also calculate the theoretical TDOA values for each pair of receivers receiving the proxy reference transmission. The processor determines the expected TDOA from the TOA of the proxy reference transmission at each of the receivers; the locations of the receivers, which are either known a priori and stored in the database or determinable from information stored in the database; and the location of the proxy reference transmitter, which is either known a priori and stored in the database or determinable from information stored in the database. The processor then either applies corrections to the wireless location system equipment to correct the offsets or applies corrections to the computed TDOA, FDOA or other parameter values, obtained during normal system operation, in the digital signal processing stages of the system.